Offshore structures, such as floating production, drilling or construction platforms or spar buoys generally are moored in a desired location through the use of chains or cables secured between the platform and anchors on the ocean floor. Typically, the practice for mooring floating platforms includes extending a chain from the ocean anchor, through a fairlead device secured to the bottom of a platform column, to chain hauling equipment and latch mechanism, such as a chain stopper, on the deck of the platform.
Mooring platforms in place over a drilling location often require the implementation of many chains, fairlead devices, anchors and chain equipment because of the massive size of the platforms. For example, the deck area of a platform is typically large enough to hold one or more buildings for housing workers and machinery, a number of cranes, and a drilling tower or limited production facilities.
Also, floatation of platforms is typically provided by a pair of large submerged pontoons. In such structures, columns are utilized, some as large as 32 feet in diameter, to support the deck on the pontoons. As a consequence of the platform's massive structure, several fairlead devices are often secured to each column of the platform and mooring chains are run through each of the fairlead devices from the anchors to chain hauling equipment on the deck.
In a typical installation, the anchor lines are installed by passing a messenger wire rope from the deck, down through the submerged fairlead, mounted near the base of the support column, and out to a pre-installed anchor chain on the ocean floor. An end connector secures the messenger wire to the anchor chain and the anchor chain is hauled back to the platform. The anchor chain passes through the fairlead and continues up to the deck. One of the requirements of an underwater fairlead is that it be able to pass the chain itself, kenter shackles, special connecting links and the wire rope installation line. On the deck, the chain hauling equipment pretensions the chain up to a predetermined percentage of the chain breaking load and then the chain stopper or chain latch, located beneath the hauling device, locks the chain in place at the pre-tensioned load.
Once the floating platform is secured in place, anchor chains are almost continuously working due to the constant movement of the platform caused by winds, waves, tides, and currents. This constant movement of the anchor chains accelerates chain fatigue failure if the chain links engage a bending shoe or sheave that has a relatively small radius, for an extended period of time. As a result, fairlead devices are typically constructed as bending shoes or sheaves that have a relatively large radius. The sheaves used in these chain mooring applications are usually seven-pocketed wheels, also known as wildcats, which cradle the chain in pockets designed to reduce the chain stresses in the links on the wildcat.
One such device is described in U.S. Pat. No. 4,742,993 to Montgomery, et al., self-aligning quadrant fairlead is secured to a platform column. The arcuate fairlead is supported by a trunnion and bearing that enables the fairlead to swing about an upright axis for self-alignment. The current disclosure in its bending shoe configuration has some similarity to the Montgomery device except that the Montgomery device was designed for wire rope and did not include an underwater chain stopper.
Another device is described in U.S. Pat. No. 5,441,008 to Lange, where a submerged swiveling mooring line fairlead device is used on a structure at sea. The fairlead is rotatably mounted in a swiveling elongated rigid tube and a chain stopper is located at one end of the elongated rigid tube. The current disclosure differs from the Lange patent because the Lange device used a tubular body connected to a separate swivel mount and the Lange device does not permit the successful passing of the wire rope, chain, center shackles and special connectors as required by the anchor chain installation schemes which are currently in practice.
Neither the Lange nor Montgomery device can be used on the chain mooring systems currently in practice. The existing technology uses a huge, seven-pocketed wildcat underwater fairlead. During installation, a messenger wire rope is fed down from the equipment deck through the fairlead. The end of this messenger wire is connected to the pre-installed anchor chain with the aid of an anchor handling ship. The messenger wire is then hauled back in thereby pulling the wire, the special connectors and the chain through the fairlead and up to the equipment deck. At the equipment deck, the anchor chain is handed off to a massive chain hauling device which is then used to pull in additional chain catenary until the desired installation tension is reached in the chain. When this tension is reached, the chain stopper is engaged and the installation is complete.
A disadvantage of the existing fairleads is their massive size. In the current technology, the chain stopper is mounted up at the equipment deck. This means that the chain is always bearing on the underwater fairlead. These chain mooring systems are always designed for loading conditions up to the breaking strength of the chain and those links which are rounding the sheave in the underwater fairlead are subjected to high stresses in the links. The links on the sheave become the weak links of the system. In an attempt to offset this problem, the industry has recently gone from five-pocket wildcats to seven-pocket wildcats to increase the bending radius of the chain. The result has been massive size, weight and increased expense for a solution which only lessens the problem, but does not truly solve it.
Another disadvantage is that when the chain stopper was stored on the deck, greater deck and column loading resulted. This condition occurred because the chain was secured to the deck through the chain stopper, which pulled down on the deck and columns. The chain stopper equipment also occupied valuable deck space and added weight to the deck.
Another disadvantage is that the submerged fairlead device is not retrievable for repair. The only means to repair the fairlead is to remove the rig from the field and take it to dry dock.
Another device is disclosed in U.S. Pat. No. 5,845,893 to Groves, the disclosure of which is incorporated by reference, is an improvement over existing devices at the time of the invention. Nevertheless, it does not include the improvements disclosed herein, such as a way to facilitate relocation of a fairlead device.